diff --git a/INSTALL.md b/INSTALL.md
index 6c01c60..e008a13 100644
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -18,6 +18,7 @@ options](#configuration) to adjust as you see fit.
     - mcrypt extension (mcrypt needs to be able to access `/dev/urandom`. This means if `open_basedir` is set, it must include this file.)
     - com_dotnet extension
 - GD extension
+- zlib extension
 - some disk space or (optionally) a MySQL, PostgreSQL or SQLite database
 - ability to create files and folders in the installation directory and the PATH defined in index.php
 - A web browser with JavaScript support